In approximately 8 percent of multiple sclerosis cases there is a positive family history. Recently there has been demonstration of an unusual association of several of the HL-A antigens with multiple sclerosis. Even more striking is the association with antigens of the mixed leukocyte culture system (MLC). A preliminary, unpublished report by Milton Alter from Minnesota suggests there may be linkage between the HL-A system and a gene or genes predisposing to multiple sclerosis. Because his study was done in a small number of families in a prescribed geographic area and did not include evaluation of MLC system, a more comprehensive study has been launched. In addition to the evaluation of Dr. Alter's preliminary finding the more general role of genetic factors in multiple sclerosis will be explored.